The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
In a laser printer, a laser beam projects onto an electrically charged, rotating drum, an image to be printed. The drum is coated with selenium, and the exposure to the laser light removes the charge from the affected areas. As the drum rotates through a supply of toner (i.e., dry ink particles), the toner is picked up by the areas of the drum that maintain a charge. The drum transfers the toner to a piece of paper by direct contact, and a fuser fuses the ink to the paper.
The fuser, the temperature of which must be accurately controlled, is heated by an AC line voltage. To control the amount of heating, the AC signal is usually gated to the fuser. When the gating signal is on, the AC waveform passes to the fuser, which generates heat. When the gating signal is off, the AC waveform does not pass to the fuser and the fuser does not heat. FIG. 1 illustrates a Triode for AC (TRIAC) 50 receiving an AC signal 52 and a gating signal 54, and outputting to a fuser 58 a gated AC signal (fuser signal) 56.
In many laser printers, the gating signal is generated using pulse width modulation (PWM). FIG. 2 illustrates a gating signal 60 generated using PWM and having a 25% duty cycle. At times 62 when the gating signal 60 is high, the TRIAC 50 passes a corresponding AC signal 64 through to the fuser. If the gating signal 60 is clocked at twice the frequency of the AC signal 64, as FIG. 2 depicts, the 25% duty cycle results in two out of every eight AC half-cycles being passed through the TRIAC 50. AC half-cycles depicted in FIG. 2 as shaded (e.g., the half-cycles 66, 68) correspond to the high periods 62 in the gating signal. FIG. 3 illustrates a pair of signals 70, 74 corresponding to the signals 60, 64 of FIG. 2. However, in FIG. 3 the duty cycle of the gating signal 70 is 50%.
A PWM device is used to generate the gating signals 60, 70. The PWM device is provided with a control signal that indicates the desired duty cycle. For example, the PWM device generates the signal 60 (FIG. 2) in response to receiving a control signal indicating a 25% duty cycle. Similarly, the PWM device generates the signal 70 (FIG. 3) in response to receiving a control signal indicating a 50% duty cycle.